Electroluminescence (EL) elements, unlike liquid crystal display apparatuses, require no backlight, allowing them to be suitable for thinner displays, and their viewing angle is not limited, there has been a growing demand for practical organic EL display apparatuses employing self-emissive organic electroluminescence (EL) element. Organic EL display apparatuses differ from liquid crystal display apparatuses employing liquid crystal cells in which display is controlled by a voltage, in that brightness of light emitted by the organic EL element used therein is controlled by the value of electric current flowing through the EL element.
FIG. 7 shows a pixel circuit in a known conventional active matrix type organic EL display apparatus. The pixel circuit includes an organic EL element 104 which is connected to a negative power source line 108 on the side of a cathode, a driver element 102 having a source electrode connected to the anode side of the organic EL element 104 and a drain electrode connected to a positive power source line 107, a capacitor 103 connected between a gate electrode and the source electrode of the driver element 102, and a switching element 101 having source and drain electrodes each connected to the gate electrode of the driver element 102 or to a signal line 105, and a gate electrode connected to a scanning line 106. Here, the switching element 101 and the drive element 102 are thin film transistors (TFTs).
The operation of the above-described pixel circuit will be described. It is first assumed that a voltage which is higher than the threshold voltage of the driver element 102 is stably stored by the capacitor 103 between the gate and source electrodes of the driver element 102. Accordingly, the driver element 102 is turned on.
In this state, the negative power source line 108 is set to a higher level than a voltage ground (GND) of the positive power source line 107. While the driver element 102 retains the on state, the potential of the anode electrode of the organic EL element 104 is made equal to the potential GND of the positive power source line 107 and a reverse bias voltage is applied to the organic EL element 104.
Then, after the potential of the scanning line 106 is set to a high level to turn the switching element 101 on, the potential of the signal line 105 is applied to the gate electrode of the driver element 102. Here, the potential of the signal line 105 corresponds to the potential GND of the positive power source line 107. This makes the potential of the anode electrode of the organic EL element 104 lower than the gate potential GND of the driver element 102 in accordance with the capacitance ratio between a capacitor component of the organic EL element 104 and the capacitor 103, causing the driver element 102 to be turned off.
Subsequently, when the potential of the negative power source line 108 is decreased to the level GND of the positive power source line 107, the potential of the source of the driver element 102 lowers in accordance with the voltage drop of the negative power source line 108, whereas the gate potential of the driver element 102 remains GND, which turns the driver element 102 on. Consequently, electric current is supplied from the positive power source line 107 through the driver element 102 to the anode electrode of the organic EL element 104, so that the potential of the anode electrode of the organic EL element 104 is gradually increased until the potential difference between the gate electrode of the driver element 102 and the anode electrode of the organic EL element 104 becomes equal to the threshold voltage of the driver element 102.
Then, the potential of the scanning line 106 is set to a low level, and the threshold voltage of the driver element 102 can be stored on the source electrode of the driver element 102 by the capacitor 103 and a capacitor component of the organic EL element 104.
Hereinafter, the process of storing the threshold voltage Vt of the driver element 102 on the capacitor 103 as described above, will be referred to as “detection of a threshold voltage”.
Then, a data voltage Vdata is supplied to the signal line 105. When the potential of the scanning line 106 is set to a high level and the data voltage Vdata is supplied to the gate electrode of the driver element 102, the potential of the source electrode of the driver element 102 changes due to a capacitance ratio between the capacitance value Cs of the capacitor 103 and the capacitance value Coled of the organic EL element 104, whereby the potential between the gate and source electrodes of the driver element 102 becomes as follows:Vgs={Cs/(Cs+Coled)}·Vdata+Vt  (equation 1)
The above potential difference Vgs is stably stored by the capacitor 103. Hereinafter, the process of adding the data voltage will be referred to as “writing”.
When the potential of the negative power source line 108 is decreased such that the potential difference between the positive power source line 107 and the negative power source line 108 is sufficiently greater than the threshold voltage of the organic EL element 104, the driver element 102 controls the electric current flowing through the organic EL element 104 in accordance with the voltage stored in the capacitor 103 by the above-described process, so that the organic EL element 104 continuously emits light with the brightness corresponding to the level of the electric current.
As described above, with the pixel circuit shown in FIG. 7, once the brightness information is written, the organic EL element 104 continuously emits light of a fixed brightness until the current writing state is cancelled (see page 2 and FIG. 1 of U.S. Published Patent Application No. 2004/0174349.)
In the pixel circuit of FIG. 7, however, at the moment in the above-described writing process when the data voltage is applied through the switching element 101, the driver element 102 turns on, as described above. Consequently, it is likely that the threshold voltage of the driver element 102 which is stored by the node between the capacitor 103 and the organic EL element 104 is lost, making it difficult to accurately superpose the information of the threshold voltage as represented by the above equation 1. In particular, as the data voltage Vdata increases and the writing time increases, the degree of threshold voltage which is lost also increases.